Transamazonian Orogeny Research Articles

Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the São Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite

New SHRIMP U-Pb zircon dating indicates that, contrary to previous thought, part of the Archean crust in the southern São Francisco Craton (SFC), NW of the Jeceaba Bom Sucesso Lineament was affected by the Paleoproterozoic Transamazonian orogeny. The Kinawa migmatite is a reworked part of the Archean crust in the southern SFC, and occurs within the Cláudio Shear Zone, a NE-NW crustal-scale structure. The protolith of the migmatite is a metagranodiorite that has an U-Pb zircon crystallization age of 2704±3Ma coeval with the voluminous potassic magmatism in the region. Moreover, the εHf(t=2704) data suggests a slightly evolved crust (−1.5 to −6.1) and thus a relatively long crustal residence (∼700Ma). Surprisingly, the migmatite itself yielded much younger ages of 2034±32Ma and 2048±25Ma, which coincide with the ages of the Mineiro Belt and Mantiqueira Complex. The distribution of ages using data from the literature and this study show that within the Archean domain the occurrence of Paleoproterozoic ages is restricted to the Cláudio Shear Zone. Inherited zircon crystals from all samples yield Neoarchean ages (∼2.7Ga), and similar Hf signatures to the metagranodiorite protolith (εHf(t=2704)=−5.5 to −0.5). The εHf(t) signatures of the Kinawa migmatite are heterogeneous and record two events, one is interpreted to be the formation of the protolith with lower 176Hf/177Hf (at ∼2.7Ga) and the other with higher 176Hf/177Hf (at ∼2.05Ga) as the anatexis that produced the migmatite. An important result is that the migmatite had two sources. Two samples are consistent with derivation from an evolved crust comparable with the metagranodiorite protolith (e.g. 176Lu/177Hf ∼0.01 and εHf(t=2034) −13.4 to −17), but the other sample has much less negative values (εHf(t=2048)=−4.6 to −8.7), which requires a different source with much lower 176Lu/177Hf, and therefore younger, possibly Post-Archean material.

The kyanite quartzite of Bosland (Suriname): evidence for a Precambrian metamorphosed alteration system

AbstractThis article investigates the origin of a rare occurrence of kyanite quartzites in the Palaeoproterozoic greenstone belt of Suriname. The rocks form elongated hills in the Bosland area, Brokopondo district, where they are associated with meta-sedimentary, meta-volcanic and granitic lithologies. Their mineral content and unusual Si- and Al-rich chemical composition are inferred to be the result of advanced argillic alteration of felsic volcanic tuffs and a later overprint by regional metamorphism up to lower amphibolite facies during the Trans-Amazonian orogeny. Structurally, the Bosland area seems centred within a contractional strike-slip duplex of a major dextral fault system. The alteration was probably associated with a high-sulphidation environment and involved significant to almost complete removal of alkali and alkaline earth elements. Pseudosection modelling and textures suggest that the precipitation–temperature (P–T) history of the kyanite quartzites started with shallow (<2kbar) hydrothermal alteration of the acidic tuffaceous volcanics, possibly in the andalusite stability field (T>350°C), and ended in peak metamorphic conditions in the kyanite–staurolite stability field (P>4kbar andT=500–650°C). Alteration events that preceded the peak of Trans-Amazonian metamorphism may be more common in the rock record of Suriname's greenstone belt, which lends support to the hypothesis that gold mineralisations in the region can be pre-orogenic.

Gold deposits of Suriname: geological context, production and economic significance

AbstractGold has been a major economic asset for Suriname for more than a century. The long history of gold mining, concentrated in large parts of a greenstone belt in the northeast of the country, began with small-scale artisanal extraction activities and has recently seen the development of major open-pit operations. Despite the range of mining activities, Suriname's gold deposits and occurrences are under-explored from a scientific point of view. Primary gold mineralisations in the greenstone belt occur in multiple forms, and although their origin is commonly related to the Palaeoproterozoic Trans-Amazonian orogeny, the controls of ore formation in specific cases often remain obscure. This contribution presents an abridged overview of currently available information on the geological setting and characteristics for some of the main deposits where gold is extracted. In view of the consistent link between gold metallogeny and granitoid–greenstone belts in the northern Guiana Shield, the mineralised settings in Suriname are discussed in a regional context.

Paleoproterozoic evolution of the Guiana Shield in Suriname: A revised model

AbstractThe Proterozoic basement of Suriname consists of a greenstone–tonalite–trondhjemite–granodiorite belt in the northeast of the country, two high-grade belts in the northwest and southwest, respectively, and a large granitoid–felsic volcanic terrain in the central part of the country, punctuated by numerous gabbroic intrusions. The basement is overlain by the subhorizontal Proterozoic Roraima sandstone formation and transected by two Proterozoic and one Jurassic dolerite dyke swarms. Late Proterozoic mylonitisation affected large parts of the basement. Almost 50 new U–Pb and Pb–Pb zircon ages and geochemical data have been obtained in Suriname, and much new data are also available from the neighbouring countries. This has led to a considerable revision of the geological evolution of the basement. The main orogenic event is the Trans-Amazonian Orogeny, resulting from southwards subduction and later collision between the Guiana Shield and the West African Craton. The first phase, between 2.18 and 2.09 Ga, shows ocean floor magmatism, volcanic arc development, sedimentation, metamorphism, anatexis and plutonism in the Marowijne Greenstone Belt and the adjacent older granites and gneisses. The second phase encompasses the evolution of the Bakhuis Granulite Belt and Coeroeni Gneiss Belt through rift-type basin formation, volcanism, sedimentation and, between 2.07 and 2.05 Ga, high-grade metamorphism. The third phase, between 1.99 and 1.95 Ga, is characterised by renewed high-grade metamorphism in the Bakhuis and Coeroeni belts along an anticlockwise cooling path, and ignimbritic volcanism and extensive and varied intrusive magmatism in the western half of the country. An alternative scenario is also discussed, implying an origin of the Coeroeni Gneiss Belt as an active continental margin, recording northwards subduction and finally collision between a magmatic arc in the south and an older northern continent. The Grenvillian collision between Laurentia and Amazonia around 1.2–1.0 Ga caused widespread mylonitisation and mica age resetting in the basement.

On the allochthonous nature of auriferous greenstones, Guayana shield, Venezuela

The tectonic style of Paleoproterozoic orogeny in the Guayana shield has been considered by most workers to be similar to those commonly invoked for the development of Archean greenstone belts, in which diapirs of granitoid magma (Supamo complex) rose and deformed 2.1–2.0Ga volcanic and sedimentary rocks of the Pastora supergroup to form keel-like greenstone belts. In this model, subsequent collision with the dominantly granulite-grade Imataca complex to the north deformed the greenstone belts during what is known as the Trans-Amazonian orogeny. However, more recent geological mapping demonstrated that the base of the greenstone belts is marked by a major mylonitic shear zone that placed rocks of the belts atop a regional unit of quartzite that sits unconformably upon granitic gneisses and plutons of both the Imataca and Supamo complexes. These relations were interpreted to indicate that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes, and as the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes predated deposition of the quartzite. New U–Pb zircon results show that volcanic rocks of the Pastora supergroup are older than the quartzite beneath them and confirm the hypothesis, based entirely upon field relations, that rocks of the greenstone belts were thrust upon the quartzite and its granitic and gneissic basement. At least two subsequent periods of deformation refolded rocks of the region. Similar structural relationships are common in Archean cratons of southern Africa, Australia, and the Canadian shield.

Supergrupo Minas na Serra de Bom Sucesso, extremo sudoeste do Quadrilátero Ferrífero - MG: petrografia, geoquímica e isótopos de U-Pb

The banded iron formation (BIF) hosted Bom Sucesso Metassedimentary Sequence (BSMS) is exposed in the southwest part of the Quadrilatero Ferrifero, Minas Gerais. This sequence has normal stratigraphic stacking and comprises three units: the lower Lagoa da Prata carbonate schist, the intermediate Bom Sucesso BIF and the upper Tabuaozinho schist. U-Pb detrital zircon data from samples of the Lagoa da Prata unit indicate a maximum age of deposition of 2603 ± 7.3 Ma, correlated with the basal Caraca Group of the Minas Supergroup. The Bom Sucesso BIF has predominant mineralogy formed by bands of iron oxide (maghemite) and amphibole; it is interlayered with discontinuous ferruginous dolomite lenses and it grades upwards to a ferruginous metapelitic layer at the top of the unit. The geochemical signature of the BIF is characterized by positive anomalies of Eu, Y and La, relative enrichment of heavy Rare Earth Elements (REE) and absence of a negative Ce anomaly, analogous to the itabirites of the Itabira Group. The mineral assemblage of the BSMS indicates contact metamorphism associated with alkaline metasomatism, generated by the intrusion of the Tabuoes Granite during the Transamazonian orogeny.

The Rhyacian El Cortijo suture zone: Aeromagnetic signature and insights for the geodynamic evolution of the southwestern Rio de la Plata craton, Argentina

The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny, rather than a single one as previously envisaged, i.e. the El Cortijo suture zone and the Salado suture. We circumscribe the Tandilia terrane to the region between these two sutures.The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane, i.e. boundary between the Tandilia and Balcarce terranes. This extensive magnetic low, ca. 300 km long, and ca. 90 km wide, would be caused by demagnetization associated with shearing. At a more local scale, the trend of the El Cortijo suture zone often turns toward the E–W. At this scale, WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian El Cortijo suture zone. Spatially associated with the El Cortijo suture zone, there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation.We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean (∼2650 Ma) crust occurred during Siderian times (2500–2300 Ma), causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by ∼2300–2200 Ma sedimentation over transitional –continental to oceanic– crust, and arc magmatism developed in the Tandilia terrane. The island arc represented by the El Cortijo Formation was also developed at this time. At late Rhyacian times, it occurred in both the closure of the narrow oceans developed previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane.

Paleoproterozoic granitoids from the northern limit of the Archean Amapá block (Brazil), southeastern Guyana Shield: Pb–Pb evaporation in zircons and Sm–Nd geochronology

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb–Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.

Mineralogical and geochemical characterization of a rare ultramafic lamprophyre in the Tandilia belt basement, Río de la Plata Craton, Argentina

A metre-thick ultramafic lamprophyre dyke intrudes the basement of the Tandilia belt at the Sierra Alta de Vela, Argentina. A petrological and geochemical study of this rock and associated small dykes indicates a predominantly calc-alkaline trend. Phlogopite K–Ar dating of the ultramafic lamprophyre gave a minimum age of 1928 ± 54 Ma as a late event of the Transamazonian Orogeny, which is well represented in the basement of the Tandilia belt.An electron microprobe study indicates the presence of phlogopite, albite, chromite and Cr-rich phenocrysts and Cr-free microphenocrysts of diopside as primary minerals. Subsequent to deformation at the contacts with the wall rock, metasomatism generated strongly zoned amphibole (edenite, pargasite, Mg-hastingite and tremolite compositions) and andradite as well as chlorite, sericite, albite, apatite and calcite. The central zone of the lamprophyre is almost undeformed and exhibits some ocellar texture. Geochemical and isotopic signatures of the lamprophyre suggest that its magma source may have previously undergone incompatible element enrichment of the mantle source, representing the original precursor magma for the calc-alkaline dyke series of the Sierra Alta de Vela.

Geochemistry, Nd isotopes and U–Pb geochronology of a 2350 Ma TTG suite, Minas Gerais, Brazil: Implications for the crustal evolution of the southern São Francisco craton

The Paleoproterozoic is considered the main period of crustal growth in the South American continent, with voluminous production of granitoid suites emplaced during the onset of the ca. 2.2–2.1Ga Trans-Amazonian orogeny. This work documents and interprets mineral chemistry, whole rock geochemistry, Nd isotopes and U–Pb geochronology data for the high-aluminium Lagoa Dourada TTG suite. This suite constitutes a hitherto unknown episode of Paleoproterozoic (ca. 2350Ma) juvenile felsic plutonism in the southern São Francisco craton, Minas Gerais, Brazil. It is also a rare example of juvenile continental crust production in the global crustal age-gap between 2.45 and 2.2Ga. The suite is composed of metaluminous to slightly peraluminous, low-Mg#, low-K and high-Ca, biotite-hornblende to hornblende-biotite (± garnet-magnetite) tonalites and biotite trondhjemites, varying from ∼62 to 73wt.% SiO2. The trace element geochemistry of the suite is characterized by mildly depleted Nd isotope compositions [ɛNd(t)=+1.0 to +2.1], low large-ion-lithophile element (LILE, i.e., Rb, Ba, and including the highly incompatible Th) and heavy Rare Earth element (REE, Yb<1.00ppm) contents. The suite is also characterized by high Sr/Y ratios (≥41 up to 81), high (La/Yb)N ratios (≥12 up to 46), and positive Eu/Eu* anomalies. These data are consistent with the derivation of the suite from a tholeiitic metabasaltic source, which had a short crustal residence time prior to melting. The chemical diversity of the suite is attributed mainly to the fractional crystallization of hornblende and accessory minerals, coupled with plagioclase accumulation in the tonalitic parental melt. Emplacement of new batches of magma could explain samples which diverge from the proposed liquid line of descent of the suite. The composition and age of the Lagoa Dourada suite suggest a link with the accretion of greenstone belt packages in an intra-oceanic tectonic setting.

Evolution of reworked Paleoproterozoic basement rocks within the Ribeira belt (Neoproterozoic), SE-Brazil, based on U–Pb geochronology: Implications for paleogeographic reconstructions of the São Francisco-Congo paleocontinent

Geological, geochemical and geochronological data suggest that the basement exposed in the Occidental terrane of the Neoproterozoic-Ordovician Ribeira belt represents the continuation of the São Francisco paleocontinent, which was amalgamated at the end of the Rhyacian (Transamazonian Orogeny). The Mantiqueira complex represents a cordilleran-type active margin of the Archean paleocontinent, while the Juiz de Fora complex represents juvenile Paleoproterozoic material, probably developed within an oceanic setting (primitive arc or plateau?). The latter unit was probably accreted to the active margin of the paleocontinent during the latest stages of the Transamazonian Orogeny, around ca. 2.05 Ma, causing the deformation and metamorphism of the Mantiqueira complex. At this time, to the west and on the cratonic foreland, concurrent deformation of the Minas passive margin and sedimentation of the Sabará basin took place. Subsequent to the Transamazonian Orogeny, the basement associations were intruded by intraplate alkaline and basic rocks related to rifting events between ca. 1.7 and 1.3 Ga that resulted in the development of intracratonic basins. The Paleoproterozoic suture located between the Mantiqueira and Juiz de Fora Paleoproterozoic terranes probably controlled the Neoproterozoic passive margin tectonics and also the development of a major thrust surface during the Neoproterozoic/Cambrian Brasiliano convergence.

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement. Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton. Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.

3.65–2.10 Ga history of crust formation from zircon geochronology and isotope geochemistry of the Quijingue and Euclides plutons, Serrinha nucleus, Brazil

The Serrinha nucleus is one of the Archaean granite-gneiss terrains of São Francisco craton, Brazil. In this area, many granitic rocks formed during the Trans-Amazonian orogeny, and intruded during and following a greenschist metamorphic event. Zircon geochronology and Nd isotopes record a history of extensive crustal reworking that began at 3.6 Ga and culminated in the emplacement and deformation of a continental arc during the Trans-Amazonian accretionary orogeny at 2.1 Ga. The basement to the arc is represented by migmatitic gneiss of the Uauá Complex, which records a period of zircon growth at 3195 ± 11 Ma but contains cores as old as 3314 ± 20 Ma, as well as metamorphic zircon as young as 3090 Ma. The Meso to Paleoarchean ages, as well as the absence of evidence for widespread Neoarchean magmatism are similar to what is observed in the Kaapvaal craton in South Africa. The Quijingue trondhjemite was emplaced into Uauá Complex migmatitic gneiss at 2155 ± 3 Ma probably during a period of TTG magmatism associated with arc construction on the edge of a Mesoarchean continent. Although older cores appear to be absent from the Quijingue trondhjemite, its depleted mantle Nd age of 3.3 Ga suggests that it was derived from partial melting of the basement. Xenocrystic zircon grains with ages as old as 3.62 Ga are present in this pluton and indicate that Eoarchean crust was present and may still be preserved in the area. Ocean convergence was probably followed by a Trans-amazonian collisional event that resulted in widespread syn to post-tectonic alkaline magmatism such as the shoshonitic Euclides pluton. Primary zircon from this pluton records an emplacement age of 2097 ± 8 Ma but the depleted mantle Nd age of 2.87 Ga indicates derivation of these magmas from Archean sources. The inherited zircon population from Euclides preserves a complex record of growth. One possible xenocryst gives a discordant and relatively imprecise age of 3654 ± 125 Ma. SHRIMP dating of other zircon grains gives concordant ages that range from 2350 Ma to 2500 Ma as well as a 2.14-Ga Trans-Amazonian component. The provenance of the very early Proterozoic ages is unknown but may indicate zircon growth or resetting events at deep crustal levels. These data combined with other results suggest that mid to early Archean continental crust could underlie much of the São Francisco craton.

Tectonic fabric revealed by AARM of the proterozoic mafic dike swarm in the Salvador city (Bahia State): São Francisco Craton, NE Brazil

Magnetic fabric and rock magnetism studies were performed on 25 unmetamorphosed mafic dikes of the Meso-Late Proterozoic (∼1.02 Ga) dike swarm from Salvador (Bahia State, NE Brazil). This area lies in the north-eastern part of the São Francisco Craton, which was dominantly formed/reworked during the Transamazonian orogeny (2.14–1.94 Ga). The dikes crop out along the beaches and in quarries around Salvador city, and cut across both amphibolite dikes and granulites. Their widths range from a few centimeters up to 30 m with an average of ∼4 m, and show two main trends N140–190 and N100–120 with vertical dips. Magnetic fabrics were determined using both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The magnetic mineralogy was investigated by many experiments including remanent magnetization measurements at variable low temperatures (10–300 K), Mössbauer spectroscopy, high temperature magnetization curves (25–700 °C) and scanning electron microscopy (SEM). The rock magnetism study suggests pseudo-single-domain magnetite grains carrying the bulk magnetic susceptibility and AARM fabrics. The magnetite grains found in these dikes are large and we discard the presence of single-domain grains. Its composition is close to stoichiometric with low Ti substitution, and its Verwey transition occurs around 120 K. The main AMS fabric recognized in the swarm is so-called normal, in which the K max– K int plane is parallel to the dike plane and the magnetic foliation pole ( K min) is perpendicular to it. This fabric is interpreted as due to magma flow, and analysis of the K max inclination permitted to infer that approximately 80% of the dikes were fed by horizontal or sub-horizontal flows ( K max < 30°). This interpretation is supported by structural field evidence found in five dikes. In addition, based on the plunge of K max, two mantle sources could be inferred; one of them which fed about 80% of the swarm would be located in the southern part of the region, and the other underlied the Valéria quarry. However, for all dikes the AARM tensors are not coaxial with AMS fabrics and show a magnetic lineation (AARM max) oriented to N30–60E, suggesting that magnetite grains were rotated clockwise from dike plane. The orientation of AARM lineation is similar to the orientation of a system of faults in which the Salvador normal fault is the most important. These faults were formed during Cretaceous rifting in the Recôncavo-Tucano-Jatobá assemblage that corresponds to an aborted intra-continental rift formed during the opening of the South Atlantic. Therefore, the AARM fabric found for the Salvador dikes is probably tectonic in origin and suggests that the dike swarm was affected by the important tectonic event responsible for the break-up of the Gondwanaland.

An epigenetic origin for the Passagem de Mariana gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The gold–tourmaline quartz–vein deposit of Passagem de Mariana, in the southeastern part of the Quadrilátero Ferrífero, produced more than 60 tonne of gold, from the end of the 17th Century until 1954. The mine has not operated since 1985. Orebodies are veins composed of white quartz (> 60 vol.%), carbonate (ankerite), tourmaline, sericite and sulfides. Tourmaline (dravite), up to 10 vol.% of the vein, occurs as subhedral, coarse, commonly zoned crystals, and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins. Tourmaline is present in all rock types in the mine, but the chemical composition of the host rocks determinates the intensity of tourmalinization, with the alteration being greater in sericitic phyllites, graphite–sericite phyllites, and calcareous rocks. The most abundant sulfide is arsenopyrite, which is normally associated with pyrite and pyrrhotite. Minor amounts of chalcopyrite, galena, löllingite, berthierite, and maldonite are present throughout the deposit. Sulfides are concentrated at veins boundary or are dispersed in the veins. Arsenopyrite is associated, most commonly, with calcareous rocks, and graphite–sericite phyllite. Pyrrhotite is usually found at the base of itabirites. Gold abundance is directly proportional to sulfide concentration. Hydrothermal alteration associated with the veins includes silicification, tourmalinization, and sulfidation. The mineralized zone is a shear zone associated with a bedding-parallel thrust fault that juxtaposes the itabirite (Lower Proterozoic Minas Supergroup) over other units. This shear zone/thrust fault extends for tens of km beyond the Passagem mine and hosts numerous gold deposits. The richest orebodies are along the itabirite footwall contact and within the graphite–sericite phyllite (Main orebody). Although many lithologic units were mineralized the graphite–sericite phyllite appears to have been most favorable for gold deposition. The area underwent three phases of deformation, D 1, D 2 and D 3. Mineral assemblages indicate upper-greenschist to lower-amphibolite conditions of regional metamorphism. Retrograde metamorphism, characterized by chloritization of biotite and chloritization and biotitization of garnet, developed locally. The gold-bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within, or are in contact with, all lithotypes. Field and laboratory data indicate that gold mineralization at Passagem de Mariana is epigenetic. Gold deposition occurred after the peak of metamorphism, within the late- to post-D 2 period of deformation, which is correlated with second set of structures of Trasamazonian age of Alkmim and Marshak [Alkmim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern São Francisco Craton Region, Minas Gerais, Brazil: evidence for Paleoproterozoic collision and collapse in the Quadrilátero Ferrífero. Precambrian Research 90, 29-58.], indicating that the gold mineralization occurred between 2.124 and 2.04 Ga. We choose to regard Passagem de Marina as an orogenic gold deposit as defined by Groves et al. [Groves, D. I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., 1998. Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews 13, 7-27.], i.e., an epigenetic, structurally-hosted lode–gold vein system in a deformed metamorphic terrane.

Geochronological framework of the Quadrilátero Ferrífero, with emphasis on the age of gold mineralization hosted in Archean greenstone belts

Archean terrains of the Quadrilátero Ferrífero comprise a greenstone belt association surrounded by granitoid–gneiss complexes, mainly composed of banded TTG gneisses whose igneous protoliths are older than 2900 Ma. This early continental crust was affected by three granitic magmatic episodes during the Neoarchean: ca. 2780 to 2760 Ma; 2720 to 2700 Ma; and 2600 Ma. Dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 Ma, contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the Quadrilátero Ferrífero. Lead isotopic studies of lode–gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 Ma. Proterozoic evolution of the Quadrilátero Ferrífero comprises deposition of a continental-margin succession hosting thick, Lake Superior-type banded iron formations, at ca. 2500 to 2400 Ma, followed by deposition of syn-orogenic successions after 2120 Ma. The latter is related to the Transamazonian Orogeny. The western part of the Quadrilátero Ferrífero was also affected by the Brasiliano Orogeny (600 to 560 Ma).

Paleoproterozoic potassic–ultrapotassic magmatism: Morro do Afonso Syenite Pluton, Bahia, Brazil

The Morro do Afonso Syenitic Pluton (MASP) is a small, bimodal suite of lamprophyric and syenitic rocks, located in the Serrinha Nucleus, São Francisco Craton, northeastern Brazil. The unit consists predominantly of alkali feldspar syenite with numerous mafic xenoliths and dykes that are predominantly lamprophyric. The syenite shows geochemical and isotopic features typical of the Roman potassic magmas, i.e. low TiO 2 (up to1.2%), low K 2O/Al 2O 3 (0.33–0.49), strong enrichment in LILE, and high LILE/HFSE ratios. Fractionation, mainly of clinopyroxene and amphibole, explains the transition from lamprophyre to syenite. Magma mixing is required to explain the most evolved compositions. MASP syenites have Paleoproterozoic crystallization ages (2.11 Ga). The Nd isotope composition of mafic and felsic phases are similar ( ε N d mafic = −2.36 to −2.61 and ε N d felsic = −2.47) and they have Archaean model ages ( T DM = 2.56–2.58 Ga) that match the age of an inherited zircon grain (ca. 2.6 Ga). MASP is similar to many other post-orogenic syenitic plutons distributed over a wide area of São Francisco Craton (SFC), which are closely comparable in age and also typically associated with ultrapotassic lamprophyre. The present results suggest that a major proportion of the potassic–ultrapotassic syenitic rocks of the SFC were emplaced in a narrow time interval from 2111 to 2080 Ma, limiting the time of metamorphism peak of the Transamazonian Orogeny to at least 2.1 Ga. These plutons show some evidence of having interactions with older felsic crustal rocks and are interpreted to be derived from partial melting of lithospheric mantle that was enriched by subduction.

Specularitic iron ores and shear zones in the Quadrilátero Ferrífero District

In the eastern high-strain domain of the Quadrilátero Ferrífero, Brazil, hard and soft high-grade hematite ore may occur as schistose bodies in shear zones mainly related to the Cambotas-Fundão thrust system. These orebodies are almost entirely constituted of oriented specularite platelets that develop both as a product of crystal–plastic deformation of hematite and solution-precipitation processes. Both mechanisms interact in the partitioning of the strain, while solution-precipitation is responsible for the precipitation of hematite and localised leaching of silica resulting in the formation of syntectonic ores. The analyses of fluid inclusions from specularite crystals of the Conceição and Timbopeba deposits, located in the northern and southernmost areas of the eastern high-strain domain of the Quadrilátero Ferrífero, were studied using infrared microthermometry and synchrotron X-ray fluorescence. The results indicate the participation of highly saline aqueous H2O–NaCl fluids containing other additional cations (Ca, Fe, K and probably Ti) in solution and homogenisation temperature Th averaging between 130 and 190°C. However, the partial fusion of metastable saturation phases indicates crystallisation temperatures of ∼400°C. The mineralisation process produces high-grade orebodies as well as Fe-rich specularitic itabirites that are explored in that area. The shear zones are related to thrust faults mapped and interpreted as related to the Pan-African/Brasiliano Orogeny that affected the Quadrilátero Ferrífero, thrusting the Paleoproterozoic platform sequence of the Minas Supergroup over Mesoproterozoic quartzites of the Espinhaço Supergroup. They intersect and even overprint an early metamorphic and tectonic fabric of Paleoproterozoic age (Transamazonian Orogeny). The geological and fabric relationships lead to the conclusion that the luids involved in the mineralisation were most probably of external origin, of still unknown source (possibly igneous), during the closure of the São Francisco Craton.

Shrimp U–Pb zircon age evidence for Paleoproterozoic sedimentation and 2.05 Ga syntectonic plutonism in the Nyong Group, South-Western Cameroon: consequences for the Eburnean–Transamazonian belt of NE Brazil and Central Africa

The Nyong Group of the NW corner of the Congo craton is a metasedimentary and metaplutonic rock unit that underwent a high-grade tectono-metamorphic event at ∼2050 Ma associated with charnockite formation. However, the age of the sedimentation and associated plutonism was not known. In view of this, the unit was considered to be part of the Archean Congo craton reactivated during a Paleoproterozoic or a Pan-African orogeny. Such interpretation was widely supported by the persistence of Archean inheritance revealed by Nd isotope data on whole rocks and U–Pb on zircons. New SHRIMP analyses on detrital zircons from metasediments (BIF, orthopyroxene gneiss and garnet gneiss) yield Mesoarchean to Paleoproterozoic ages, with the youngest zircon at 2423 ± 4 Ma, thus giving the maximum deposition age for the Nyong Group. Data on a metagranodiorite at Bonguen and a metasyenite at Lolodorf yield emplacement ages of 2066 ± 4 Ma and 2055 ± 5 Ma respectively, with Archean inheritance (2836 ± 11 Ma) for the metasyenite. The syntectonic emplacement of these plutonic rocks is supported by the age of 2044 ± 9 Ma obtained on the Bienkop charnockite, associated with Eburnean high-grade metamorphism which continued probably up to 1985 ± 8 Ma. These new data permit correlation of the Nyong rocks with the Paleoproterozoic of NE Brazil and the discussion of the source provenance of detritus for the Nyong Group. Finally, it is proposed that the West Central African Belt (WCAB) in southern Cameroon, Gabon, Congo and Angola represents a segment of the Eburnean–Transamazonian orogeny that resulted from the convergence and collision between the São Francisco-Nigerian Shield block and a former Congo megacraton.

Structural evolution and tectonic setting of the Porongos belt, southern Brazil

The SW–NE-striking Porongos belt, located between juvenile Neoproterozoic rocks in the west and the Dom Feliciano belt, characterized by intense reworking of older crust, in the east, comprises a greenschist to amphibolite-facies metavolcano-metasedimentary succession (Porongos sequence) of unknown age with some exposures of Palaeoproterozoic gneisses (Encantadas gneisses). High-temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing (DT1–DT3) and can be attributed to the Palaeoproterozoic Trans-Amazonian orogeny. The deformation of the Porongos sequence occurred during the Neoproterozoic Brasiliano orogeny and comprises four ductile deformation phases (DB1–DB4), including two phases of isoclinal folding associated with shearing recorded in mylonitic layers, followed by closed NW-vergent folding and thrusting leading to formation of a thrust stack. Uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi-ductile to brittle block faulting in a sinistral strike-slip regime. The Porongos sequence can be subdivided into a southeastern and a northwestern part. Trace element analyses as well as Sm–Nd and Rb–Sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks. The metavolcanic rocks show εNd(t=780 Ma) values of −20.64 and −21.72 (northwestern units) and −6.87 (southeastern sequence). The metasedimentary rocks were derived from late Palaeoproterozoic to Archaean sources, and the data indicate different sources for the northwestern and southeastern rock units of the Porongos sequence. εNd(t=780 Ma) are −6.25 and −6.85 in the southeastern units, with TDM model ages between 1734 and 1954 Ma, and vary between −14.72 and −17.96 in the northwestern parts, which have TDM model ages between 2346 and 2710 Ma. High 87Sr/86Sr(t) values between 0.7064 and 0.7286 confirm reworking of older crust. Isotopic signatures of the Porongos sequence do not show indications for a significant contribution from a Neoproterozoic juvenile source. A passive margin or continental rift environment is suggested for the tectonic setting of the Porongos belt, which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust.